We have learned, or are in the process of finding out, a good deal about the cytological changes occurring in the lower auditory system of rats as they pass from young adulthood to very advanced age. In those instances in which comparable observations have been made in human clinical material from cases of presbycusis, striking similarities to the aging picture in rats are seen, e.g., hair cell loss, or strial degeneration. The present work builds upon our knowledge of normally occurring aging changes by beginning to explore experimentally, in the rat, some of the variables that may be of significance in the genesis, time course, and severity of age-related hearing losses. The proposed studies, which represent the first systematic efforts in this direction in the area of presbycusis, are designed to investigate the role of: long-term history of elevated sound intensities, long-term history of reduced sound intensity, and degree of melanin pigmentation. In each ease, the aim is to determine the influence of the variable on the aging processes affecting specific cell group of the lower auditory system, including the auditory receptor cells of the cochlea. Experimental analysis of the three variables will primarily involve study of the following animals: aged animals which have been regularly exposed to wide-band noise of high but not deafening intensity throughout their adult lifespans; aged animals which underwent surgical interruption of the ossicular chain in the middle ear (incudectomy) at one month of age and whose adult level of auditory stimulation is therefore reduced in intensity; and aged animals which are of genetically related strains but which differ in degree of melanin pigmentation. The study of auditory cells in the experimental animals will be carried out quantitatively, with principal reliance upon the techniques of scanning and transmission electron microscopy. By contributing to an understanding of the factors important in the development of presbycusis, the proposed studies will aid in defining environmental and occupational areas in which preventative efforts may prove fruitful.